Question

I’m working on a ruby script that ultimately starts up a system process that takes quite a while. I need to read from the stderr of this process and react to it depending on what is output.

I’m currently doing it as such:

Open3.popen3(cmd_to_run) do |stdin, stdout, stderr, waitthread|
  stderr.each_line do |line|
    # look out for specific lines and react to them accordingly
  end
end

But I’ve also seen implementations to achieve something similar but doing it with kernel#select:

Open3.popen3(cmd_to_run) do |stdin, stdout, stderr, waitthread|
  io = select([stderr], nil, nil, 30)

  if io.nil?
    log("Command timed out during Kernel#select")
    return
  end

  io[0][0].each_line do |line|
    # look out for specific lines and react to them accordingly
  end

end

I’ve read the pickaxe description of what select does, but I’m confused as to why I should (of if I should) use it? The first method works just the same.

Answer 1

Probably two reasons:

1. You can use timeout, which you can’t with each_line
2. You can wait for more than one IO object, e. g. io = select([stdout, stderr]) and more than one event (e.g. write event or exception too)